The first technician that came over thought the problem was the condenser capacitor, which was old, I replaced that but didn't fix the problem. All the components were running, including the compressor and both fans inside and outside but no cooling. He said the refrigerant pressure was OK but...

The second guy said the refrigerant was completely gone (which means the first guy was lying and will never call him again) and thought the leak must be withing the evaporator coils based on a known problem with Lennox that even has a class action lawsuit. But that was a red flag to me because he just wanted to replace the coils on a mere suspicion and no definitive leak find. So I won't be calling that guy anymore either.

I brought a third guy who did a nitrogen test but could not find any leaks. We sprayed all the brazed connections with microleak detection fluid, no bubbles anywhere, no hissing noise, no nothing. Both on the condenser side as well as in the coils. He said basically just recharge the system. Here is the pressure gauge readings that held for like 45 min.

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I have enough leftover 410a in a tank from last time the compressor was changed, so it would not be expensive to just wing it and recharge to see what happens. But the test results are not consistent with the incident of losing all the refrigerant in the system. So is there anything else that could explain this mysterious loss of refrigerant?

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    That, (which should cause them to lose their US-EPA license to handle refrigerants) Or the leak is from where the gauges are connected to.
    – Ecnerwal
    Commented May 12, 2023 at 18:42
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    @Ecnerwal, I assume you refer to the last sentence, which I deleted as paranoid, as your "that"
    – amphibient
    Commented May 12, 2023 at 18:46
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    If it isn't the lineset then you're pretty much sol. It's in one of the coils. I guess you could try some leak stop, but that's new system time. Pressure testing with an analogue meter? Don't call them back either. Nitrogen test is if it holds pressure (because it's benign and what you have on hand to do the soldering), and when to crack the vales open for the first time, not to find a leak. Need to put some in the system and use a sniffer. Leak stop is BS though. You either fill this system every year or replace it.
    – Mazura
    Commented May 12, 2023 at 20:13
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    "I have enough leftover 410a in a tank from last time the compressor was changed" Just a note : you can put your refrigerant back in the unit. They can't w/o having it "recovered". .... Also, what? "from last time the compressor was changed".... it's done, bro. Finito. New unit time.
    – Mazura
    Commented May 12, 2023 at 20:23
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    With "the last time it was changed", I was assuming it was a recovery tank. They're not supposed to be charging a system with a leak. But you don't have any certification to jeopardize.... using a little bit to find a leak is better than just arbitrarily charging a system every year. Hooking a gauge set to it shouldn't completely drain the system, unless the schrader vales are bad. If they didn't remove them when they soldered then they're compromised.
    – Mazura
    Commented May 12, 2023 at 20:34

1 Answer 1


This doesn't explain the loss of refrigerant, but is another troubleshooting tool.

There's another type of leak test that may be better than the N2 test. You need to have a running system to use this. They add a pound or so of some sort of dye to the system, and let the system run - the longer the better. Then the tech comes back with a UV (I think) light and looks for any signs of the dye (that fluoresces under UV).

Only shortcoming with this test is that if the leak is outside in a place exposed to rain, any precipitation will tend to wash any dye off.

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    You have to sniff or look for something, and it isn't nitrogen, +1.
    – Mazura
    Commented May 13, 2023 at 0:00
  • Have you checked the compressor and the service valves, they can leak as well. Be sure to check the compressor terminals where they come out of the can.
    – Gil
    Commented May 13, 2023 at 0:22

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